I downloaded Aqua's, Korone's, Okayu's and Shion's mmd models and had a look. akuokakoro had decent rigs while Shion had a simple rig. the other three had more control on more parts of the body. I'm not saying Shion's is bad but it's weird that hers was that simple compared to the other three. generational gap isn't even an excuse since Aqua's was the most complex out of all of them. here's a concrete example. the other three had a bone called "weapon" so that they can attach an object to it so that they can hold shit. Shion doesn't have that, at least the one on
mmd.hololive.tv doesn't. I assume they updated what they use but not what's on the site, which is weird since Miko recently had a model put there.